101 Building Physics & Condensation or why is it important to open your windows?

Have you ever wondered why there is water running down your window? Or why you have damp spots or even mould in your bathroom or behind the robe? No, it does not come from the outside. (except of course if there is some sort of water leakage somewhere).

The main sources of moisture in a home are cooking, baking, but also all other processes where water is used, like having a shower or a bath, using the toilet, washing your hands or the dishes, using the dishwasher or the washing machine, indoor planting and open water features, like aquariums or indoor ponds and pools. But did you know that one of the main contributors to the moisture in the air and in your home are we humans ourselves?

You might be asking: why, it can’t be that bad. How much moisture, water can there be? Here a little example.

Let’s take a small 3-person household, living in a 100square metres home (While this sounds tiny for most Australians, this is the average home size in Germany!) The indoor temperature is 20° C and 65% relative humidity (which is kind of average). This means there is about 2,8 litres water in the air. In average, this household will produce about 12 litres of water every day.

Between 2-3 litres by breathing, 4 l by showering and washing, about 4 litres for cooking and about 2-3 litres for indoor plants and the like. This 12 litres does not include any extra humidity due to drying clothes inside , which would increase the amount even further.

Summarising this means each household produces each month between 300 and 600 litres of water that is converted into steam. Only a portion of this water will be extracted through ventilation directly. (This is if you open the windows). The rest will be stored initially in the construction elements (walls, ceiling and floor) and furnishings and then later on extracted indirectly. Sufficient fresh air must be provided to exchange the ‘used’ air to allow for the required direct and indirect extraction of the moisture content in the air.

Let’s go one step back. Why is there moisture in the air?

Air is a mixture of different gases. From our school days, we know the air consisting of nitrogen (N), oxygen (O ²) and carbon dioxide (CO ²) as the main constituents.

The absorption of water (H ² O) depends on the temperature of the air. Warm air can hold more water than cold air.

Condensation occurs when air with accumulated moisture content through climate and occupancy cools down.

If the temperature drops, the moisture content of the air remains the same initially, but the maximum capacity to hold water will be reduced. As a consequence the relative humidity is increased. Once it has cooled down to a point where the existing humidity reaches the saturation value condensation takes place. The relative humidity is 100%.

The temperature at which this condition occurs is called the dew point temperature; this depends on the moisture content as well as the temperature of the air.

If temperatures on glazing, walls, ceilings, floors, windows are colder than the room temperature this can lead to condensation on the surface of the materials, which, if not tried out, can lead to mould and fungees. But it is also possible that condensation occurs within the construction, for instance within the insulation. This condensation within the wall components can also be harmful in certain circumstances to the structural integrity of the entire home.

In Europe homes are sealed so tightly that we are taught how important it is to open the windows at least a couple of times a day to exchange the used air. Well, here in Australia most people haven’t heard about it. But when you live in an old ‘typical’ Australian home, this might not be an issue, as there are so many gaps and openings everywhere, that fresh air might get in anyway. But if you live in a new home, or you are thinking about renovating and making your home more energy efficient by sealing your house, this is an entirely different story. Condensation and hence mould can occur internally which can lead to several health issues, like asthma, eczemas, etc., if you do not ventilate your home properly.

However, keeping the rising energy prices in mind and our environment in general, there is nothing more important than making our homes more energy efficient. At least that’s my opinion. In order to do this we have to insulate our houses better, seal it as air tight as possible and also avoid thermal bridges. (I will explain more about thermal bridges in another article).

The better insulated and the more air-tight our homes get, the more we have to convert our thinking. The temperature inside the house, the air’s moisture content as well as the temperature on the surfaces inside the house is fluctuating constantly and depends largely on the usage by its occupants, as well as heating and cooling habits.

It will not be enough anymore to open the window every now and then. You have to exchange the air inside by opening the windows regularly, ideally twice a day for let’s say 5 minutes and create a proper cross-ventilation. Just opening one awning window won’t do it.

 

Humidity levels for typical household processes can be specified as follows:

Emitted amount of water vapour in the household (per person/per day)

Human body – light to medium activity –                   1 to 1,5 litres

Human body – while sleeping –                                     1 litres

Shower                                                                                 1 – 1,5 litres

Bathing                                                                               0.5 – 1 litres

Cooking                                                                              0.5 – 1 litres

Dishwasher (one load)                                                    0,2 litres

Washing machine (one load)                                         from 0.2 to 0.3 litres

Drying tumble dry washing inside (4.5 kg)                 from 1 to 1.5 litres

Drying wet washing/clothing inside (4.5 kg)            2 to 3.5 litres

Flowers                                                                             from 0.5 to 1 litres

Indoor plants from                                                        1 to 1.5 litres

Water features (eg pond, aquarium)                          from 0.8 to 1.3 litres.

 

If you have a really air tight home it is actually recommendable to ventilate the house 4 – 5 times a day properly and perform a complete air exchange.

Now you might think, gosh, that will just increase my heating bills. But, consider this. It’s actually the other way around. Firstly, the higher the humidity inside your house, and inside the wall, the higher the thermal conductivity of the wall, meaning the more heat can the wall store and detract from the room, meaning you have to heat more. But even more critical is, that the insulative properties of the insulation will get reduced significantly. As a rule of thumb you can say that for each 1% moisture increase inside the insulation  value drops by 5%. That’s actually quite a lot, isn’t it?

Summary

It can be stated that a dwelling with insufficient ventilation will most likely have higher heating bills due to the poorer performance of the thermal insulation. It is not recommended to open for instance one awning window the entire day. This will in fact just cool down the house constantly and not exchange the indoor air. Ideally you should open one casement window or door on either side of the house to create a good cross ventilation for about 5 minutes.

Having said all this, this applies for highly energy efficient, highly insulated and well-sealed homes. However, keep in mind even if you home is not that air tight, the humidity is there, even when you can’t see it, and opening the windows should become a daily habit of yours.